Multi-dimensional manipulation of optical field withmetasurfaces and its optimization based on machine learning

Abstract

Metasurfaces, as two-dimensional artificial subwavelength nanostructures, have shown novel optical phenomena and abilities of flexible and multi-dimensional optical field manipulation with a more integrated platform. Relative to the metasurfaces with single-dimensional manipulation, the metasurfaces with multi-dimensional manipulation of optical fields show significant advantages in various practical application areas, such as optical holograms, sub-diffraction imaging, generating vector optical fields, and so on. However, the design of metasurfaces with multi-dimensional manipulation of optical fields is more complex in most cases. Optimization based on machine learning can effectively lower the difficulty of the design and achieve more accurate multi-dimensional manipulation, which has attracted great interest in recent years. In this review, we firstly classified and discussed the recent advances of two-dimensional manipulation of optical fields with metasurfaces. Then, we further introduced the multi-dimensional manipulation method of optical fields based on machine learning. We first provide a following classification and review recent works related to two-dimensional manipulation of optical fields with metasurfaces. (1) Manipulating amplitude and phase of optical fields simultaneously. In scalar optics, the transmission of complete optical field information requires both amplitude and phase modulations, as the wave equation implies. Thus, the metasurfaces with both amplitude and phase manipulation can provide full optical field information and better performance in various areas, such as optical holograms, energy-tailorable metasurfaces, generating Airy beams and bidirectional perfect absorbers. (2) Manipulating phase and polarization of optical fields simultaneously. In vectorial optics, many intriguing phenomena occur which seem impossible in scalar optics. However, in conventional optics, complex optical systems including polarizers and curved mirrors are often required to achieve simultaneous manipulation of phase and polarization. Metasurfaces with both phase and polarization manipulation can be utilized to achieve various novel functionalities, such as generating vector beams, giant optical activity without chiral structures, chiral holograms, arbitrary spin-to-orbital angular momentum conversion of light and anisotropic coding metasurfaces, with an integrated optical system. (3) Manipulating amplitude and polarization of optical fields simultaneously. Utilizing the abundant interlayer effects provided by metasurfaces, one can tailor their reflection, transmission, and absorption properties of electromagnetic waves in different polarization states. Metasurfaces with both amplitude and polarization manipulation can be utilized to achieve various novel phenomena which are difficult to be realized in conventional optics, such as asymmetric transmission and giant circular dichroism. (4) Manipulating frequency and amplitude of optical fields simultaneously. In linear optical region, the unique frequency-dependent absorption and scattering properties of metasurfaces in the visible frequencies are utilized to print color at the nanoscale, which is known as structural colors. In nonlinear optical region, the exponential relationship between the intensities of the nonlinear optical signal and the fundamental frequency signal, many optical phenomena can be further amplified, such as circular dichroism. (5) Manipulating frequency and phase of optical fields simultaneously. In linear optical region, achromatic meta-lenses can be realized by utilizing the dispersion characteristics of different nano antennas to compensate for phase dispersion at different frequencies. In nonlinear optical region, metasurfaces with efficient phase modulation of nonlinear waves have received great attention in recent years for their great value in the realization of many novel nonlinear functionalities, such as multiplexed holograms, multiplexed coding meta-devices, and terahertz generators. (6) Multi-dimensional manipulation of on-chip optical field. With the integration with the waveguides, metasurfaces can also realize multi-dimensional manipulation of on-chip optical field. Metasurfaces consisting of phased nanoantennas can be used to control guided waves via strong optical scattering at subwavelength intervals. Different on-chip meta-devices have been demonstrated based on this design principle, such mode converters, spin-selective and wavelength-selective demultiplexing devices, polarization emitters, waveguide devices supporting asymmetric optical power transmission and phase-matching-free second harmonic generator. Then, we introduce the new multi-dimensional manipulation method of optical fields based on machine learning. The manipulation method based on machine learning can quickly obtain the optimal structural parameter solution in a huge parameter space, which improves the efficiency of parameter optimization exponentially. At the same time, machine learning can be used to design complex discrete structures, which breaks the period limitation of conventional devices. The optical field manipulation with metasurfaces based on machine learning has great application value in metalens, holographic imaging, coding metasurface and so on.